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Application & OT Security · Intermediate

Certificate Transparency (CT)

Public, append-only logs of issued TLS certificates that let anyone monitor which certificates exist for a domain.

Definition

CT (RFC 6962) was designed to detect misissued certificates, and browsers require public CAs to log certificates. Because the logs are public, attackers and testers search them (for example via crt.sh) to discover subdomains and internal hostnames. Organizations should monitor CT for unauthorized certificates.

Example

Searching %.example.com on crt.sh reveals vpn.example.com and dev-api.example.com.

Exam tip

CT is passive recon and also a defensive control for detecting rogue certificates.

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